The invention relates to a data transmission method in a system which utilizes the CDMA method, in which several users communicate simultaneously on the same frequency band, and in which each user has at least one broad-band information channel, and in which the information channel capacity of at least one user differs from the capacity of the other users.
CDMA (Code Division Multiple Access) is a multiple access method, which is based on the spread spectrum technique and which has been applied recently in cellular radio systems, in addition to the prior FDMA and TDMA methods. CDMA has several advantages over the prior methods, for example spectral efficiency and the simplicity of frequency planning.
In the CDMA method, the narrow-band data signal of the user is multiplied to a relatively wide band by a spreading code having a considerably broader band than the data signal. In known test systems, bandwidths such as 1.25 MHz, 10 MHz and 25 MHz have been used. In connection with multiplying, the data signal spreads to the entire band to be used. All users transmit by using the same frequency band simultaneously. A separate spreading code is used over each connection between a base station and a mobile station, and the signals of the users can be distinguished from one another in the receivers on the basis of the spreading code of each user.
A CDMA receiver comprises means, which can be implemented for example with correlators or matched filters, for synchronization with a desired signal, which is recognized on the basis of the spreading code. In the receiver, the data signal is restored to the original band by multiplying it again by the same spreading code as during the transmitting stage. Signals multiplied by some other spreading code do not correlate in an ideal case and are not restored to the narrow band. They appear thus as noise with respect to the desired signal. The spreading codes of the system are preferably selected in such a way that they are mutually orthogonal, i.e. they do not correlate with each other.
A CDMA system according to the background art is disclosed in U.S. Pat. No. 5,166,951, which is incorporated herein by reference and which describes a high-capacity system wherein the spreading code sequences used in the information channels are mutually orthogonal.
In a typical mobile phone environment, the signals between a base station and a mobile station propagate along several paths between the transmitter and the receiver. This multipath propagation is mainly due to the reflections of the signal from the surrounding surfaces. Signals which have propagated along different paths arrive at the receiver at different times due to their different transmission delays. CDMA differs from the conventional FDMA and TDMA in that the multipath propagation can be exploited in the reception of a signal. One way of realizing a CDMA receiver is to use for example a so-called rake receiver, which consists of one or more rake branches. Each branch is an independent receiver unit, the function of which is to compose and demodulate one received signal component. Each rake branch can be caused to synchronize with a signal component which has propagated along an individual path, and in a conventional CDMA receiver the signals of the receiver branches are combined advantageously, for example coherently, whereupon a signal of good quality is obtained. The signal components received by the receiver branches may be transmitted from one base station, or in the case of macrodiversity, from several base stations.
The present cellular systems are mainly designed for the transmission of conventional speech traffic. This is also true for the latest digital systems, such as the European GSM system. There have been efforts to also enable the transmission of data services in digital systems, but so far the services and possibilities provided therein have been of inferior quality than in the fixed network.
An aim in developing cellular radio systems is to better take into account the needs of different data services. An essential factor in the transmission of services is that different services require transmission channels with considerably different capacity and quality requirements. The requirements set for both transmission rate and bit error ratio, for example, may vary depending on the type of service. For example, the transmission of video image over the radio path of the cellular system requires considerably more capacity than the transmission of speech.
An obvious manner of solving the problem of implementing connections requiring different amounts of capacity is to utilize the present speech channels flexibly so that connections requiring a high capacity are provided with several channels. In TDMA systems this often means several time slots, and in CDMA systems it means that a subscriber terminal simultaneously transmits and receives transmissions provided with several spreading codes. However, this is a very simple manner of implementing a system with multiple data rates or quality criteria, and it wastes capacity.
The purpose of the present invention is to provide a method of implementing multiple data rates or quality criteria in such a way that the quality of signals remains good and that the capacity of the system will be efficiently utilized. A further purpose of the present invention is to implement a transmitter that is capable of transmitting the signal of the user in an information channel having the desired capacity. Another purpose of the present invention is to implement a receiver that is capable of receiving signals with different types of waveforms and of eliminating the effect of interfering signals from desired transmissions.
This is achieved with the method of the type described in the preamble, characterized in that the parameters of each information channel depend on the desired capacity and transmission quality, and that each information channel is detected by taking into account the correlations between the channels.
The invention also relates to a receiver in a system which utilizes the CDMA method, in which several users communicate simultaneously on the same frequency band, and in which each user has its own spreading code, and in which the information channel capacity of at least one user differs from the capacity of the other users, the receiver comprising converter means for converting a received transmission into a digital form, and a number of means for synchronizing and for calculating the characteristic required for a decision from the transmission of each received user. The receiver according to the invention is characterized in that said number of means each comprise a group of correlators which may be synchronized with waveforms of different types, and that the receiver comprises means for processing said characteristics calculated for the different types of waveforms by eliminating the effect of their mutual correlations.
The invention also relates to a transmitter in a system which utilizes the CDMA method and in which several users communicate simultaneously on the same frequency channel, the transmitter comprising means for forming the symbols to be transmitted. The transmitter according to the invention is characterized in that the transmitter comprises means regulating the length of the symbols to be transmitted, means for converting a desired number of symbols to be transmitted into a parallel form, means for generating spreading code sequences the number of which equals the number of the symbols to be transmitted in parallel, and means for multiplying each of the symbols to be transmitted by its own sequence.
In the method according to the invention, different data transmission needs can be implemented together with a high transmission quality. Each system user may have different parameters of the transmission connection, i.e. the type of the waveform in the information channel used, according to the requirements set for the connection. In this connection, connection parameters refer to for example the chip rate or length of the spreading code and to the data symbol length.
In a preferred embodiment of the invention, different users have data symbols of different length. The data symbol lengths of different users are selected in such a way that the symbol lengths used are fractions of the length of a specific given super symbol. The spreading code sequences that modulate the data symbols of the users recur at intervals of the given super symbol. During detection the interference between the users can be decreased by taking into account the mutual correlations between the received signals. Detection can be implemented advantageously in such a way that a desired number of signals are received and correlation terms over the super symbol to be detected are calculated, and these correlation terms are utilized in the detection in such a way that the mutual interference between the signals to be transmitted with different data symbol rates can be eliminated.
In a second embodiment of the invention, which can be used as a part of the above-described embodiment, a user transmits some of his data symbols in a parallel form so that each parallel symbol is multiplied by a spreading code sequence that is formed of parts of a longer spreading code. This spreading code is used over connections requiring a lower capacity to multiply one symbol. In a third embodiment of the invention, the spreading codes by which the parallel data symbols are multiplied are substantially mutually nonorthogonal. In the receiver the signals naturally interfere with one another, but the interference can be eliminated during the detection by means of a suitable interference cancellation algorithm.